Method of improving the collapse strength of conduits

ABSTRACT

Method of improving the hydrostatic collapse strength of conduits which have undergone rotary straightening comprising passing the conduit through a rotary straightening apparatus one or more additional times.

[4 1 Oct. 15, 1974 METHOD OF IMPROVING THE COLLAPSE STRENGTH OF CONDUITS [75] Inventor: Parviz Mehdizadeh, Ponca City,

Okla;

[73] Assignee: Continental Oil Company, Ponca City, Okla.

[22] Filed: Sept. 4, 1973 [21] Appl. No.: 394,202

[52] US. Cl. 72/367 [51] Int. Cl B21d 3/02 [58] Field of Search 72/78, 98, 110, 367

[56] References Cited UNITED STATES PATENTS 2,178,141 10/1939 Framed 72/367 R16,220 12/1925 Mirfield Primary Examiner-Lowell A. Larson Attorney, Agent, or Firm-Gerald L. Floyd [5 7 ABSTRACT Method of improving the hydrostatic collapse strength of conduits which have undergone rotary straightening comprising passing the conduit through a rotary straightening apparatus one or more additional times.

7 Claims, No Drawings p 1 METHOD or IMPROVINGVTHE COLLAPSE STRENGTH or CONDUITS' I BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of improving the hydrostatic collapse strength of conduits, particularly those conduits which have undergone rotary straightening operations. 7

2. Description of the Prior Art In the manufacture of rigidmetal conduits, especially those having a substantial length to' diameter ratio, there is often a tendency for some segments of the conduit to be crooked; i.e., not axially'straight. To correct this defect, the crooked sectionsof conduit undergo a rotary straightening operation-.In this operation, the conduit segment is passed between a plurality of rollers at least some of which exert pressureon the exterior of the conduit, bending it, and axially straightening it. While such an operation desirably improves the axial straightness of the conduit, it also has the undesirable effect of decreasing the hydrostatic collapse strength of the conduit. i I 1 In many operations in which conduit is ultimately utilized, it is important that the conduit be capable of withstanding considerable hydrostatic pressure without collapsingif the higher pressure is on the outside of the conduit or without bursting if the greater pressure is on the inside of the conduit.

It is an object of this invention to provide a method.

of improving the hydrostatic collapse strength of metal BRIEF SUMMARY OF INVENTION A process'for increasing the hydrostatic collapse strength of a metal conduit which has been passed through a rotary straightening apparatus comprising passing the so-straightened conduit throughthe same or a different rotary straightening apparatus more additional times.

IDESCRIPTIONOF THE PREFERRED EMBODIMENT The process of this invention can be utilized on any metal conduit. .Of particular interest are steel tubing and casing such as that installed in water and petroleum wells. Such tubi'ngfand casing are manufactured and cut into sections which are up to 13 inches or more in diameter'and'often 30 feetor more in length. As manufactured, they are'sometimes'axially crooked and must be straightened before they are suitable for use. Of the various straightening operations available, rotary straighteing is one of the more effective .and 'quick methods. However, conduit that is passed through a rotary straightening apparatus loses a substantial portion of its hydrostatic collapse strength. This loss can be detrirnental to many conduits. It is especially onerous with at one end of the apparatus through which a segment 2' well tubing and casing which must be able to withstand a substantial pressure differential in use.

Any common type of rotary straightening apparatus may be used in the process of this invention. Theexact design of the apparatus is immaterial. Such apparatus in general comprise: a first cluster of two or three rolls of conduit to be straightened is fed, a second cluster of two or three rolls at the opposite end of the apparatus through which the segment of conduit is received and passes, and at least one pressure-adjustable roll intermediate the two cluster of rolls which concurrently and progressively deflect the conduit transversely of its axis.

During the 'straighteningioperation, the conduit may be at any temperature from ambient temperature to the tempering temperature of the conduit metal.

EXAMPLES 1-3 A series of three identical tests were made to determine the collapse strength of fully quenched unstraightened 7 /s-inch diameter, 26.4 pound N-80 grade quenched well casing An open-end hydraulic compression chamber was clamped around the specimen. The specimen was contained at each end of the chamber by a perforated front ring, cup gasket, and a backup ring. The tightening and packing device provided a pressure-tight system for the hydrostatic external pressure test.'Air was then pumped into the annular space between the specimen and the compression chamber while the pressure was observed on a pressure gauge. The pressure was increased until the specimen collapsed. The pressure was immediately released and the maximum reading recorded as the collapse pressure. The collapse .pressure was corrected with respect to the average ratio of diameter of the casing to wall thickness of the casing (D/t) divided by the nominal ratio of diameter of the casing to wall thickness of the casing (D'lt). The nominal D'lt' for 7 %-inch diameter, 26.4 pound N-8O grade casing is 23.247. The results of these collapse tests are shown in Table I below.

EXAMPLES 4-6 A series of three other identical tests were made to.

determine the collapse strength of the same N-80 driven roll and two idler or friction-driven pressure rolls positioned at approximately to each'other. Straightening was accomplished by feeding a segment of easing through the twothree-roll clusters while pressure was being applied by the middle unopposed roll.

A majority of-the straightening occurs due to the bending of the casing by the unopposed middle pressure roll.

A minor amount of straightening occurs due to the rolling action of the casingin the cluster rolls. After this straighteningoperation, each segment of casing was tested in the collapse strength tester as described-in connection with Examples 1 to 3. The results of these testsare shown in Table I below. The results show a decrease in'the collapse pressure compared to the un- EXAMPLES 7-9 A series of three other identical tests were made to determine the collapse strength of the same N-80 grade casing which had been fully quenched and straightened as described in Examples 4-6 above and, in addition, had been passed through the same rotary straightening apparatus a second time. This procedure illustrates the process of the instant invention. The results of these collapse tests are shown in Table 1 below. The results show a collapse pressure, while not as high as that of the unstraightened casing of Examples 1 to 3, which is considerably higher than the casing of Examples 4 to 6 which had been passed one time through the rotary straightening apparatus. Thus, a substantial portion of the collapse strength which was lost by one straightening operation was regained by carrying out a second straightening operation.

TABLE I LIGHT WEIGHT HYDROSTATIC COLLAPSE PRESSURE OF N-80 GRADE CA'SlNG* '7 SIS-inch diameter. 26.4 pound N-80 grade casing EXAMPLES 10-13 These Examples are similar to Examples 1-3 except that 33.7 pound N-80 grade casing was used rather than 26.4 pound casing. The nominal ratio of diameter of the 33.7 pound casing to the wall thickness of the casing is 17.733. Collapse strength was determined as above on fully quenched unstraightened specimens. The results of these tests are given in Table 11 below.

EXAMPLES 14-17 These Examples aresimilar-to Examples 4 to 6 except that 33.7 pound -N-80 grade casing was used rather than 26.4 pound casing. Collapse strength was determined as above on fully quenched specimens which had passed through the rotarystraightening apparatusone time. The results of these tests are given in Table 11 below. The results show a decrease in the collapse pressure compared to the unstraightened casing of Examples 10 to 13.

EXAMPLES 18-2 These Examples are similar to Examples 7 to 9 except that 33.7 pound N-80 grade'casin g was used rather than 26.4 pound casing. Collapse strength was determined as above on fully quenched specimens which had been passed two times through the rotary straightening apparatus. These examples represent a practice of the process of this invention. The results of these tests are given in Table 11 below. The results show a collapse strength which is substantially higher than either the unstraightened casing of Examples 10 to 13 or the casing of Examples 14 to 17 which had been passed one time through the straightening apparatus.

TABLE 11 HEAVY WEIGHT HYDROSTATIC COLLAPSE PRESSURE ()F N- GRADE CASING 7 5l8-inch diameter. 33.7 pound N-80 grade casing While applicant does not want to be held to any particular theory as to why the process of this invention results in conduit having improved collapse strength, it is believed the improvement results from two factors: an improvement in dimensional uniformity; i.e., uniform diameter and wall thickness, and a more uniform distribution of residual stresses in the conduit metal.

The reason that a greater improvement in collapse strength occurs with relatively heavy thick-walled 33.7 pound N-8O grade casing compared to relatively light thin-walled 26.4 pound casing is not well understood. It is postulated that when collapse occurs in the 26.4 pound casing, the metal is in the elastic collapse region whereas when collapse occurs in the 33.7 pound casing, the metal is in the plastic collapse region.

1 claim:

l. A method of improving the hydrostatic collapse strength of metal conduit which has undergone a first v rotary straightening treatment comprising subjecting said metal conduit to a second rotary straightening treatment.

2. The method of claim 1 wherein the metal conduit is steel conduit.

3. Themethod of claim 2 wherein the steel conduit is tubing or casing used in completing water and petroleum wells.

4. A method of improving the hydrostatic collapse strength of metal conduit which has undergone rotary straightening by being passed one time through a rotary straightening apparatus comprising passing said metal conduit through a rotary straightening apparatus a second time.

ening apparatus comprises an apparatus having a plurality of rolls which bear against the metal conduit during its passage through said apparatus with sufficient force to straighten the metal conduit. 

1. A method of improving the hydrostatic collapse strength of metal conduit which has undergone a first rotary straightening treatment comprising subjecting said metal conduit to a second rotary straightening treatment.
 2. The method of claim 1 wherein the metal conduit is steel conduit.
 3. The method of claim 2 wherein the steel conduit is tubing or casing used in completing water and petroleum wells.
 4. A method of improving the hydrostatic collapse strength of metal conduit which has undergone rotary straightening by being passed one time through a rotary straightening apparatus comprising passing said metal conduit through a rotary straightening apparatus a second time.
 5. The method of claim 4 wherein the metal conduit is steel conduit.
 6. The method of claim 5 wherein the steel conduit is tubing or casing used in completing water and petroleum wells.
 7. The method of claim 4 wherein the rotary straightening apparatus comprises an apparatus having a plurality of rolls which bear against the metal conduit during its passage through said apparatus with sufficient force to straighten the metal conduit. 